This disclosure relates to electronic devices and more particularly, to pulse-density-modulated (PDM) analog-to-digital (A-D) and digital-to-analog (D-A) conversion systems.
A-D conversion and D-A conversion can be used for subsonic, sonic, and ultrasonic frequency conversion and across diverse applications such as audio, video, seismology, vibrational analysis, materials science, acoustic microscopy, biomedicine, phased arrays, earth sciences, and many forms of test equipment.
A-D conversion is typically performed by a device known as an analog-to-digital converter (ADC). D-A conversion is typically performed by a device known as a digital-to-analog converter (DAC). Analog signals are commonly converted and represented digitally by a multi-bit conversion format called pulse-code modulation (PCM). Each PCM sample is represented by a discrete binary digital number, for example, 8, 16, 24, 32, or any other number of bits. PCM conversion is commonly realized using integrated circuits, and less commonly via discrete topologies, such as discrete R2R resistor ladder.
Analog signals are less commonly converted and represented digitally by a single-bit format generally known as PDM. In a PDM-represented analog signal, the relative density of the pulses corresponds to the analog signal amplitude. In a PDM bit-stream, a “1” (or “high”) corresponds to a pulse of positive polarity and a “0” (or “low”) corresponds to a pulse of negative polarity. A bit-stream consisting of all 1s would correspond to the maximum (positive) amplitude value, all 0s would correspond to the minimum (negative) amplitude value, and equally alternating 1s and 0s would correspond to a zero amplitude value. An example of a PDM digital-audio delivery format is sold by Sony Corporation under the registered trademarks, DIRECT STREAM DIGITAL® and DSD®.
In conventional PDM conversion topologies, a single dynamically-contiguous analog signal is converted by an ADC configured for PDM conversion, such as AK5572 by Asahi Kasei Microdevices, and represented in PDM data as a single, contiguous bit-stream. The PDM bit-stream can be reconverted into a single, dynamically-contiguous analog signal by a DAC configured for PDM operation, such as an AK4490 by Asahi Kasei Microdevices.
Two operational parameters of ADCs and DACs are self-noise and maximum level. The difference between self-noise and maximum level is called dynamic range. Historically, the dynamic range of audio-frequency ADCs and DACs has improved at roughly 0.7 decibels (dB) per year, on average, with three fundamental technical breakthroughs representing the majority improvement of historic dynamic range performance: electric recording and reproduction, magnetic tape recording and reproduction, and digital-audio recording and reproduction.
Improvements in ADC and DAC dynamic range can be found in both PCM and PDM conversion. However, the dynamic range performance of some aspects of PDM conversion have generally not kept pace with PCM conversion performance.